For the first generation, the number of larvae per plant is determined by the percentage of plants infested and average number of larvae per infested plant found in the field samples. After scouting a field as described in the Scouting Techniques section, use the cost-benefit analysis chart to determine if control measures are economically justi-fied. This same analysis is in the European corn borer micro-computer software (Iowa State University version only). The following example demonstrates the use of the analysis chart in Table 5.

A producer's field is scouted in the 10th-leaf stage, and 450 European corn borer larvae are found in 100 corn plants. About two-thirds of the larvae are in the second instar, and the environmental conditions are considered to be moderate. Consequently, survivorship is estimated to be 0.40 (40 percent). Therefore, 180 larvae out of the 450 found are expected to bore into 100 plants later in development. Larvae per plant is calculated at 1.8. Since the yield loss for field corn in the 10th-leaf stage of development is expected to be 5.9 percent per larva, the proportion of yield loss per larva is 0.059. The proportion of yield loss from the European corn borer larval population found in the field is 0.106, which is the yield loss calculation. This means that 10.6 percent of the yield could be lost to European corn borer, if the field goes untreated. The producer has averaged 140 bushels per acre over the past 10 years and wants to use this average for analysis on the field. The predicted bushels lost per acre is 14.84, based on the proportion of yield loss and expected yield. At $2.50 per bushel, the value of corn that would be lost per acre is $37.10. If treatment is required, the producer predicts that the proportion of larvae controlled by applying a spray in the whorl would be 0.8 (80 percent). Multiplying this number by the loss per acre equals $29.68 preventable loss that can be saved by applying a control. Subtracting the cost of control from the preventable loss will give the amount of money either gained or lost, if the control is applied. Cost of control includes both the application cost andinsecticide cost for one application. In this example, if the cost of control is $14.00, there is a net gain in profit of $15.68 per acre estimated for applying an insecticide to this field.

aAll percents must be written using decimals (i.e., 50 percent = 0.5). bExpected survivorship usually is 0.2 to 0.5 (20 to 50 percent), depending on the instars and degree of environmental stress on the larvae before they bore into the plant. cUse 0.059 for 10-leaf corn, or 0.05 for 16-leaf, whorl-stage corn.